In general, continuous ink jet printing apparatus have a print head manifold cavity to which ink is supplied under pressure so as to issue in streams from a print head orifice plate that is in liquid communication with the cavity. Periodic perturbations are imposed on the liquid streams, e.g. vibrations by an electromechanical transducer, to cause the streams to break up into uniformly sized and shaped droplets. A charge plate, comprising an array of addressable electrodes, is located proximate the break-off points of the streams to induce an electrical charge, selectively, on adjacent droplets, in accord with print information signals. Charged droplets are deflected from their normal trajectory; e.g. in one common (binary) printing mode, charged (non-print) droplets are deflected into a catcher device and non-charged droplets proceed to the print medium.
A number of different catcher devices have been developed as constructions to intercept and recirculate the non-print droplets from such print heads. The catcher devices must take several potential problems into account. First, the catcher device must intercept the non-print ink droplets in a way that avoids splattering them onto the print medium, or scattering into an ink mist, which also can cause defects on the print media. Second, the catcher devices must effectively remove the caught ink away from the droplet interception zone so that a build-up of ink in the catching surface does not block the flight path of printing drops.
Where the catcher is moving during the print operation or where the droplet stream is not vertical (so that ink in the discharge channel is subjected to transverse gravitational forces) efficient collection of caught ink can be frustrated. Also, when the catcher is part of a moveable print head assembly, acceleration forces can cause ink at its catcher throat to be slung away from the catcher. Slung ink masses can appear on the print media as defects or contaminate the machine. Even where the acceleration forces are not sufficient to sling the ink, they can cause dynamic buckling of the ink film just entering the catcher throat. The buckled ink film can obstruct ink droplets which should pass to the print media, which cause splatter and/or "white defects," as a result of the droplet interception.
A solution to the above problems is proposed in U.S. Pat. No. 5,105,205 which describes a continuous ink jet catcher device having a screen in the catcher throat for improved flow control of caught ink. Referring to FIG. 9, a partial cross-sectional schematic view of the prior art catcher assembly generally designated 18 includes a catcher body 30 defining a catcher face 20, a catcher throat 22 and a screen 48 in the catcher throat. The screen 48 is folded as shown in FIG. 9 and inserted into the catcher throat 22 after construction of the catcher 18. To be effective, the screen 48 in the catcher throat 22 must be held flush to the catcher face 20 and must encompass the entire throat opening 22. Experience has shown that the prior art construction suffers from the lack of means to positively locate the screen. The screen has been observed to shift positions in the throat and render the screen ineffective. In worst cases, the screen 48 has fallen out of the catcher assembly. Furthermore, through routine handling during the manufacturing process or handling of the print head during installation and removal, the screen shape can become compressed to the point of being rendered useless.